A crystalline silicon-based solar cell in which a conductive amorphous silicon-based layer having a band gap different from that of a single-crystalline silicon is formed on a surface of the single-crystalline silicon substrate is called heterojunction solar cell. Among heterojunction solar cells, a solar cell having an intrinsic amorphous silicon thin-film between a conductive amorphous silicon-based layer and a single-crystalline silicon substrate provides a crystalline silicon-based solar cell with a high conversion efficiency.
In crystalline silicon-based solar cells such as heterojunction solar cells, tetragonal pyramid shaped (pyramid-shaped) irregularity structures called textures are often formed on a surface of a crystalline silicon substrate. When textures are formed on a substrate surface, light incident to a crystalline silicon substrate is scattered, and therefore the optical path length in the crystalline silicon substrate increases, so that current density of a solar cell is increased.
After formation of textures on a surface of a crystalline silicon substrate and before formation of an amorphous silicon-based layer on the crystalline silicon substrate, several kinds of treatments are performed in order to adjust a texture shape and clean the surface. For example, Patent Document 1 and Patent Document 2 disclose that after pyramid-shaped textures are formed on a surface of a crystalline silicon substrate by anisotropic etching and before a surface cleaning treatment is performed, the substrate is immersed in an alkali or an oxidizing aqueous solution to isotropically etch the substrate surface, so that sharp portions of textures are removed to round valley portions (recess portions). Patent Document 2 proposes a method in which a substrate is immersed in ozone water, and then immersed in a hydrofluoric acid aqueous solution to remove an oxide film formed by ozone oxidation as a method for cleaning a silicon substrate after formation of textures.